The low molecular weight Cd binding proteins collectively referred to as metallothionein are clearly involved in the response of animals to exposure to heavy metals, and especially to Cd. The precise toxicological function of Cd-metallothionein remains unclear, however. The present proposal deals with attempts to test critically some of the suggested functions. These include a role of Cd-metallothionein in detoxification, storage, transport and excretion of Cd. For this purpose labelled Cd-metallothionein will be injected into normal an Cd poisoned rabbits, and the kinetics of its renal reabsorption, accumulation and excretion will be determined. The results anticipated from such experiments may not only help clarify a critical aspect of Cd poisoning, but also contribute to an understanding of the mechanisms of renal excretion of heavy metals.